Triplet Repeat Disease

Triplet repeats were once thought to be apparently benign stretches of DNA. However, it is now known that these repeats can sometimes undergo dynamic or expansion mutation. In this type of mutation, through mechanisms during DNA replication that are only partly understood, the number of triplets in a repeat increases (expands) and can cause disease.

There are many diseases known to be caused by triplet repeats. They share certain common features. The mutant repeat length is unstable in both somatic and germ cells of the body, meaning it can change in length during DNA replication. Also, the triplet repeat often expands rather than contracts in successive generations. Increasing repeat size is correlated with decreasing age of onset or increasing disease severity in successive generations. This phenomenon is called anticipation, and is a characteristic of most triplet repeat diseases.